1. Field of the Invention
This invention relates to photoreceptors suitable for use in electrophotography and, more specifically, to photoreceptors having novel overcoats comprising a urethane-acrylic resin and a rheology modifier.
2. Background of the Art
In electrophotography, a photoreceptor in the form of a plate, belt, disk, sheet or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas. This pattern is referred to as a latent image. A liquid or solid toner is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoreceptor. The resulting visible toner image can be transferred to a suitable receiving medium such as paper and film, or the photoreceptor surface can operate as a permanent receptor for the image. The imaging process can be repeated many times when a temporary or intermediate receptor is used.
The photoconductive element can be organic or inorganic. Both single layer and multilayer photoconductive elements have been used. In the single layer embodiment, a charge transport material and charge generating material are combined with a polymeric binder and then deposited on the electrically conductive substrate. In the multilayer embodiment, the charge transport material and charge generating material are in the form of separate layers, each of which can optionally be combined with a polymeric binder, and deposited on the electrically conductive substrate. Two arrangements are possible. In one arrangement (the “dual layer” arrangement), the charge generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the “inverted dual layer” arrangement), the order of the charge transport layer and charge generating layer is reversed.
A photoreceptor is required to have desired sensitivity and electrical properties depending on an electrophotographic process applied thereto. A photoreceptor subjected to repetitive uses is also required to have an excellent durability and abrasion-resistance against chemicals including the carrier fluid in the toners, electrical forces, and mechanical forces applied thereto during corona charging, toner development, transferring to a receiving medium, and cleaning treatment. Furthermore, the surface layer of the photoreceptor may be contaminated by toners, and therefore it should have a good release property. Lastly, the surface of the photoreceptor should have good electroconductive properties so that charge will not remain on the surface of the photoreceptor after discharge to cause a background problem on prints.
For the surface layer of a photoreceptor to possess the above-mentioned desirable properties, the photoreceptor may be provided with an overcoat to protect the photoconductive element. The typical overcoats comprise fluorinated polymer, silicone or siloxane polymer, fluorosilicone polymer, polyethylene, polypropylene, polyurethane, polycarbonate, polyester, acrylated polyurethane, acrylated polyester, acrylated epoxide resin, or a combination thereof Although these overcoats provide reasonable abrasion-resistance and durability, they may not be good enough for the recent requirement of further improved image quality. This invention provides a protective overcoat layer having further improved properties in respects of chemical, mechanical, and electroconductive properties.